realtek: use vendor-specific magic for ZyXEL

realtek: use vendor-specific magic for ZyXEL

The stock firmware of the ZyXEL GS1900 series use a non-standard
u-image magic.  This is not enforced by the stock u-boot, which is
why we could boot images with the default magic.  The flash
management application of the stock firmware will however verify
the magic, and refuse any image with another value.

Convert to vendor-specific value to get flash management support
in stock firmware, including the ability to upgrade to OpenWrt
directly from stock web UI.

Signed-off-by: Bjørn Mork <bjorn@mork.no>